Method for controlling an electronically commutated dc motor

ABSTRACT

A method of controlling an electronically commutated DC motor ( 10 ) having a multiphase stator winding ( 11 ) which has an even number m of winding phases ( 111 - 114 ) which are each connected in series with a controllable semiconductor switch ( 12 ), parallel to one another, in which in a lower power output range of the DC motor ( 10 ) the semiconductor switches ( 12 ) are cycled using a pulse control factor which is predefinable as a function of the rpm within the consecutive periods when the individual winding phases ( 111 - 114 ) are energized. In order to reduce the maximum power losses in the semiconductor switches ( 12 ), a setpoint pulse control factor required for a setpoint rpm is achieved within a selected setting range of the pulse control factor by alternatingly setting a comparatively greater pulse control factor and a comparatively lower pulse control factor, and the setting of the two pulse control factors is varied over time so that a voltage setting the setpoint rpm (n setp ) is set across the stator winding ( 11 ) on the average over time.

FIELD OF THE INVENTION

[0001] The present invention is directed at a method of controlling anelectronically commutated DC motor.

BACKGROUND INFORMATION

[0002] In an electronic control circuit for an electronically commutatedDC motor (EC motor) having a three-phase stator winding and a permanentmagnet-excited rotor as referred to in German Published PatentApplication No. 43 10 260, three semiconductor switches configured asMOS FETs are each connected in series with a winding phase of the statorwinding, and the three series circuits are connected in parallel. Thecontrol terminals of the semiconductor switches receive commutatingsignal-triggered control signals, which depend on the rotor position, sothat the semiconductor switches may be triggered using a current flowangle, definable by the length of the control signals, within thecommutating angle (block control). In order to avoid undesired featuresof block control in the lower rpm range (occurrence of high currentpeaks at low motor speeds and noisy operation), in a lower rpm segmentat 100% triggering, the amplitude of the control signal is increasedwith increasing rpm up to a first rpm (linear control), then withincreasing rpm up to a second rpm the amplitude of the control signalsis increased to a maximum and at the same time the degree of triggeringof the current flow angle (block length) is reduced from 100% to a lowervalue. Then, starting at the second rpm, with increasing rpm up to themaximum rpm, the degree of triggering is increased again from the lowervalue to 100% with maximum amplitude of the control signals. The pureblock control in the upper rpm range may avoid the lower efficiency,which may be inherent in linear control.

[0003] In variable-speed EC motors, also known as brushless DC motors,there are motor topologies where the power loss in the semiconductorswitches does not increase with the motor output, but rather is higherin the partial load range than in the full load range. This may beundesirable in particular for drives whose intrinsic cooling increaseswith increased power output of the DC motor, such as for example in pumpmotors, which are cooled by the medium pumped. Such motor topologies maybe encountered, for example, in EC motors having a single-strand ormultistrand, even-numbered multiphase winding, for example, adouble-strand four-phase winding or a three-strand six-phase winding.These EC motors may be controlled in cycled operation by pulse-width(PW) modulation. With increased pulse control factor of the cycle, i.e.,switched-on time of the semiconductor switch in relation to the cyclelength, the power losses in the semiconductor switches increase morethan proportionally, so that such EC motors are not operated in thecyclic mode in the upper motor output range, and the motor output ischanged by varying the block length via the block control. In this case,the time periods in which the individual winding phases are energizedincreasingly overlap. In this type of control, the maximum power loss inthe semiconductor switches may occur shortly before the transition fromcycled operation to block operation.

SUMMARY OF THE INVENTION

[0004] An exemplary method according to the present invention mayprovide that certain pulse control factors required for the desired rpm,which cause a high power loss in the semiconductor switches may beavoided, and the setpoint rpm may be achieved by varying, over time,those pulse control factors which cause lower power losses in thesemiconductor switches. In this manner the maximum losses in thesemiconductor switches may be effectively reduced in the partial loadrange, and the efficiency of the DC motor may be improved. This mayresult in reduced cooling requirements for the semiconductor switches,for which smaller heat sinks may now be sufficient, which in turn mayresult in space and cost savings.

[0005] The exemplary method according to the present invention mayrequire no additional hardware. All control measures affecting thecommutating signals may be implemented by software modules using theexisting hardware. Overall, the exemplary method according to thepresent invention may result in increased efficiency and reduced cost inthe manufacturing of the EC motors in question.

[0006] According to one exemplary embodiment of the exemplary method,the smaller and greater pulse control factors may be selected so thatthe power loss in the semiconductor switches-is lower with these pulsecontrol factors than the power loss occurring at the setpoint pulsecontrol factor. The setpoint pulse control factor may be achieved byalternating between the two pulse control factors, the frequency ofalternation between the pulse control factors being adjusted to theconfiguration characteristics of the DC motor, for example, its momentof inertia. Thus, the two pulse control factors may be set consecutivelywhile a winding phase is energized, or the two pulse control factors maybe switched after half of an electrical turn, a full turn, or multipleturns of the motor.

BRIEF DESCRIPTION OF THE DRAWING

[0007]FIG. 1 shows a block diagram of an EC motor having an electroniccontrol.

[0008]FIG. 2 shows a diagram of the power loss of the semiconductorswitches in the EC motor as a function of the EC motor rpm.

[0009]FIG. 3 shows a diagram of the control signals for thesemiconductor switches in each winding phase for three different controlmodes.

DETAILED DESCRIPTION

[0010] In the exemplary embodiment illustrated in FIG. 1 as a blockdiagram for elucidating the exemplary method according to the presentinvention, an electronically commutated DC motor, referred tohereinafter as EC motor 10, is set, i.e., regulated at a predefinablesetpoint rpm n_(setp). EC motor 10 has a two-strand, multiphase statorwinding 11 having an even-numbered m, here m=4, winding phases 111-114and a rotor 15 excited by a permanent magnet. Winding phases 111 and112, as well as 113 and 114 of each winding strand are wound in oppositedirections and inductively coupled. Each of winding phases 111-114 isconnected in series with a semiconductor switch 12, which is configuredin this case as a MOS-FET. The four series circuits each composed of oneof winding phases 111-114 and one semiconductor switch 12, together witha capacitor 13, are connected into a parallel circuit, which isconnected to a DC system 14, the point of common coupling of the fourwinding phases 111-114 being connected to the positive pole of DC system14 and the point of common coupling of semiconductor switches 12 beingconnected to frame potential.

[0011] Winding phases 111-114 are also connected to a commutating device16, in which the voltages induced in winding phases 111-114 are furtherprocessed. Furthermore, in commutating device 16, an rpm signal isgenerated from the induced voltages which corresponds to actual rpmn_(act) of EC motor 10 and is applied as an rpm-proportional DC signalto a comparator 17, configured for example as a differential amplifierand which also receives setpoint rpm n_(setp). Comparator 17 comparessetpoint rpm n_(setp) and actual rpm n_(act), and the difference issupplied to an rpm regulator 18. The regulator output signal is appliedto the input of a pulse width modulator 19. Pulse width modulator 19generates a separate control pulse train for each winding phase 111-114;these control pulse trains are gated with the commutating signals incommutation device 16. Semiconductor switches 12 of the individualwinding phases 111-114 are triggered by the control signals obtainedfrom these logic operations, so that each semiconductor switch 12 iscycled with an rpm-dependent pulse control factor when energized. Thecycling of semiconductor switches 12 determines the magnitude of thedirect voltage applied to EC motor 10, i.e., its stator winding 11, andthe rpm is varied by varying this voltage; the nominal torque may befully taken into account at all rpm levels.

[0012] In an EC motor 10 thus controlled, the power loss ofsemiconductor switches 12 increases with increasing pulse controlfactor, i.e., pulse width divided by the pulse period, in other words,with increasing pulse width modulation. Therefore, the motor control viapulse width modulation is restricted to the lower half of the outputpower spectrum in cycled operation, and in the upper half the motoroutput is varied by block control, specifically by increasing the anglewhere each winding phase is energized beyond the commutating angle. Inthe exemplary embodiment of the four-phase EC motor according to FIG. 1,the commutating angle is 90 electrical degrees. The control mode isswitched for semiconductor switches 12 at an rpm n_(b) that is much lessthan the idling rpm, which is achieved using a 100% pulse widthmodulation when the winding is energized over an angle of 90 electricaldegrees.

[0013]FIG. 2 shows power loss P of semiconductor switches 12 as afunction of rpm n of EC motor 10. It may be seen that power loss Pincreases suddenly shortly before reaching rpm n_(b), i.e., shortlybefore the transition from cycled control to block control. In theexample of FIG. 2, the maximum-power loss P occurs at a pulse controlfactor or pulse width modulation of 95%. In order to reduce this powerloss and thus to improve the efficiency of EC motor 10, the followingcontrol method may be used in pulse-width modulator 19 for generatingthe control signals for semiconductor switches 12:

[0014] A certain set range of pulse control factors, in which the powerloss in semiconductor switches 12 exceeds a predefined value at anypulse control factor, is selected within the pulse control factors thatmay be produced by pulse width modulator 19. In the example of FIG. 2,this set range is selected between a pulse width modulation or pulsecontrol factor of 80% and a pulse width modulation or pulse controlfactor of 100%. For these two pulse width modulations or pulse controlfactors, the power loss in semiconductor switches 12 is about the same,while in the range of pulse width modulations or pulse control factorsin between the power loss of semiconductor switches 12 always assumes ahigher value. If a setpoint pulse control factor which is located inthis selected range is required due to a required setpoint rpm n_(setp),i.e., in the example between a pulse width modulation or pulse controlfactor of 80% and a pulse width modulation or pulse control factor of100%, a greater and a smaller pulse control factor is set compared tothis setpoint pulse control factor, both of which are located outsidethe selected set range, and the two pulse control factors are variedover time so that a voltage which corresponds to the voltage producedusing the setpoint pulse control factor and which controls the rpm atsetpoint rpm n_(setp) is obtained across stator winding 11 on theaverage over time. For example, the smaller pulse control factor is setat 80% and the greater pulse control factor is set at 100%, and thesetting is varied as appropriate over time. The frequency of variationbetween the two pulse control factors is adjusted to the configurationcharacteristics of EC motor 10, e.g., its moment of inertia, and thevariation may be performed in different ways.

[0015] In order to achieve an average voltage across stator winding 11,which may require a pulse width modulation or pulse control factor of90% and may set the desired setpoint rpm n_(setp), FIG. 3 shows threedifferent options for varying the two pulse control factors. In allthree examples, the smaller pulse control factor is 80% and the greaterpulse control factor is 100%. FIGS. 3a and 3 b show, for each windingphase 111-114, one period of a commutating signal which is applied tothe respective semiconductor switch 12 during one 360 electrical degreesrevolution of rotor 15. FIG. 3c shows a plurality of periods of thecommutating signals.

[0016] In the example of FIG. 3a, the setting of the two pulse controlfactors is varied so that the reciprocal value of the frequency ofvariation between the two pulse control factors corresponds to theenergized time period of a winding phase 111-114, i.e., the time duringwhich a winding phase 111-114 is energized (during a revolution of rotor15 by 360 electrical degrees), the 100% pulse control factor and the 80%pulse control factor are consecutively set, so that each semiconductorswitch 12 in a winding phase 111-114 is cycled with a fictitious pulsecontrol factor of 90% on the average, while the resulting power loss insemiconductor switches 12 is merely the average power loss between thesubstantially lower power losses at a pulse control factor of 80% and apulse control factor of 100%. The energized time of a winding phase111-114 is calculated from the constant energized angle of winding phase111-114 which for the assumed four-phase winding 11 amounts to 360electrical degrees divided by four, i.e., 90 electrical degrees, takinginto account the rpm of EC motor 10. For example, if a setpoint rpmn_(setp) is to be set which requires a voltage across EC motor 10 whichis to be set using a pulse control factor or a pulse width modulation of95% and would result in the maximum power loss in semiconductor switches12, then within the energizing time of individual winding phase 111-114,the time segment in which the respective semiconductor switch 12 istriggered using the lower pulse control factor of 80% is reducedaccordingly, so that on average a fictitious pulse control factor of 95%is obtained. As the broken line in the diagram of FIG. 2 shows, theincreased power loss in the range between a pulse control factor of 80%and a pulse control factor of 100% is considerably reduced and does notexceed the power loss produced at a pulse control factor of 80% insemiconductor switches 12.

[0017] In the example of FIG. 3b, the setting of the two pulse controlfactors of 80% and 100% is varied so that the reciprocal value of thefrequency of variation between the two pulse control factors correspondsto half a revolution of EC motor 10. Thus, winding phases 111 and 113are triggered using a pulse control factor of 100%, and phases 112 and114 are triggered using a pulse control factor of 80%, so that on theaverage a fictitious pulse control factor of 90% is obtained, wherebythe power loss is reduced as described above. A fictitious pulse controlfactor of 95% may be achieved, for example, by cycling winding phases111, 112, and 113 using a pulse control factor of 100%, and windingphase 114 using a pulse control factor of 80%. The power loss thusobtained corresponds on the average to a power loss obtained at a pulsecontrol factor of 80% or a pulse control factor of 100% and may thus beconsiderably lower than the power loss that would be obtained with apulse control factor of 95%.

[0018] In the example of FIG. 3c, the setting of the 80% pulse controlfactor and of the 100% pulse control factor is varied in order toachieve a fictitious pulse control factor of 90% so that the reciprocalvalue of the frequency of variation between the two pulse controlfactors corresponds to a full electrical revolution of EC motor 10. Inthis case, in successive revolutions of EC motor 10 each winding phase111-114 is triggered alternatingly with a pulse control factor of 100%and a pulse control factor of 80%.

[0019] The reciprocal value of the frequency of variation between thetwo pulse control factors 80% and 100% may, however, also correspond toa multiple of an electrical revolution of EC motor 10. Thus, forexample, each winding phase 111-114 may be triggered with a pulsecontrol factor of 100% during two electrical revolutions and with apulse control factor of 80% during a third electrical revolution. On theaverage, triggering with a fictitious pulse control factor of 95% wouldresult, which would still produce a lower power loss in semiconductorswitches 12 than by triggering each semiconductor switch 12 with theactual pulse control factor of 95%.

[0020] The present invention is not limited to the above-describedexemplary embodiment of a double-strand, four-phase EC motor 10. Thesame control method also may be used, for example, for an EC motorhaving a three-strand, six-phase stator winding, in which likewise thewinding phases of a winding strand, wound in opposite directions, areinductively coupled and therefore have the motor topology as describedabove for EC motor 10.

What is claimed is:
 1. A method of controlling an electronicallycommutated DC motor (10) having a multiphase stator winding (11) whichhas an even number m of winding phases (111-114) which are eachconnected in series with a controllable semiconductor switch (12),parallel to one another, in which in a lower power output range of theDC motor (10) the semiconductor switches (12) are cycled using a pulsecontrol factor, which is predefinable as a function of the rpm, withinthe consecutive periods when the individual winding phases (111-114) areenergized, wherein a setpoint pulse control factor required for asetpoint rpm is achieved within a selected set range of the pulsecontrol factor by alternatingly setting one pulse control factor that isgreater than the setpoint pulse control factor and one pulse controlfactor that is smaller than the setpoint pulse control factor, and thesetting of the two pulse control factors is varied over time so that avoltage which regulates that rpm at the setpoint rpm (n_(setp)) is setacross the stator winding (11) on the average over time.
 2. The methodas recited in claim 1, wherein the smaller pulse control factor and thegreater pulse control factor are selected so that the power loss in thesemiconductor switches (12) at these pulse control factors is less thanthe power loss obtained at the setpoint pulse control factor.
 3. Themethod as recited in claim 1 or 2, wherein the selected set range of thepulse control factor is the range in which the power loss obtained inthe semiconductor switches (12) for each pulse control factor exceeds apredefined value.
 4. The method as recited in claim 3, wherein a pulsecontrol factor of 100% is selected as the greater pulse control factorat which the switched-on time of the semiconductor switches (12) isequal to the switching period.
 5. The method as recited in claim 3 or 4,wherein a pulse control factor of 80% is selected as the smaller pulsecontrol factor at which the switched-on time of the semiconductorswitches (12) is equal to 80% of the switching period.
 6. The method asrecited in one of claims 1 through 5, wherein the frequency of variationbetween the pulse control factors is adjusted to the designcharacteristics of the DC motor (10), for example, its moment ofinertia.
 7. The method as recited in claim 6, wherein the reciprocalvalue of the frequency of variation between the two pulse controlfactors corresponds to the time during which a winding phase isenergized.
 8. The method as recited in claim 7, wherein each pulsecontrol factor occurs at least once for an energizing time segmentappropriately defined, during the time when a winding phase (111-114) isenergized within an electrical revolution of the DC motor (10)
 9. Themethod as recited in claim 6, wherein the reciprocal value of thefrequency of variation between the two pulse control factors correspondsto one-half of an electrical revolution of the DC motor (10).
 10. Themethod as recited in claim 9, wherein the pulse control factor is variedin consecutive winding phases (111-114) during an electrical revolutionof the DC motor (10).
 11. The method as recited in claim 6, wherein thereciprocal value of the frequency of variation between the pulse controlfactors corresponds to at least one electrical revolution of the DCmotor (10).
 12. The method as recited in claim 11, wherein each of thetwo pulse control factors is maintained unchanged in all winding phases(111-114) during an electrical revolution of the DC motor (10), and thepulse control factor is varied in each nth electrical revolution, nbeing an integer equal to or greater than
 2. 13. The method as recitedin one of claims 1 through 12, wherein the time during which the windingphases (111-114) are energized is derived from an energized angle whichis calculated by dividing electrical 360° by the even number m ofwinding phases (111-114) and is constant in each winding phase(111-114), taking into account the motor rpm.